Biomolecules include small biomolecules, which have relatively small molecular weights, such as sugars (sugar chains), nucleosides, hormones, vitamins, and so forth. These small biomolecules, which frequently lack a characteristic light absorption spectrum, are hard to be detected and identified by only so-called spectroscopic methods.
As means for detecting the small biomolecule, can be contemplated an method utilizing an interaction between such a small biomolecule and another substance. For example, Patent Document 1 discloses a method of detecting an interaction between a protein and a small molecule, which method includes analyzing a structural change in a protein produced by the incorporation of a small molecule at the protein's active site, for example, NMR, and determining the structure of the small molecule with a mass spectrometer. Patent Document 2 discloses a method that uses a surface-localized plasmon resonance (SPR) sensor, which method includes solid phase-bonding a sample on a thin metal film formed on a substrate and detecting the degree of absorption of specific incident laser light on the sample, by use of the SPR.
However, the method disclosed in Patent Document 1 requires a substantial number of large and expensive instruments, i.e., a liquid chromatograph, NMR, and mass spectrograph. On the other side, the method disclosed in Patent Document 2 requires the immobilization of an auxiliary molecule on the metal surface and the detection at high reproducibilities requires an extremely high degree of technique.
A fluorescent method, though being known as simple and highly sensitive methods for detecting the interaction between nucleic acid and protein, are unsuitable for the detection of the small biomolecules referenced above. The fluorescent method includes labeling both of the interacting substances (for example, nucleic acid and protein) with different fluorescent dyes which interact with each other to enable the fluorescence to be changed, and measuring whether the change actually occurs; however, it is quite difficult to label small molecules with these fluorescent dyes. Even if it can be done, the fluorescent dye may exert such an influence on an active site as to interfere with a normal reaction.    [Patent Document 1] Japanese Patent Application Laid-open No. 2006-226880    [Patent Document 2] Japanese Patent Application Laid-open No. 2005-283566